if module was marked as not existing by setting sys.modules [fullname] to None, then pkgutil.get_loader (fullname) will throw AttributeError.

Example:
#! /usr/bin/evn python
import unittest
import pkgutil

def main ():
    pkgutil.get_loader ('unittest.functools')

if __name__ == '__main__':
    main ()

Patch is attached

So I'm no pkgutil expert, but at least in the case of None in sys.modules, that triggers at least an ImportError in __import__ if that is come across.

Main use case of pkgutil.get_loader is to determine if its possible to load module and either return loader or None. But it throws exception more over it is AttributeErrror exception.

I'm not yet sure the proposed fix in the patch is the right approach (I need to look at the surrounding code), but I believe Pavel's right that get_loader() should be returning None in this case instead of throwing an exception.